SVG renderers should share a base class, like how CSS Box model renderers do
RenderSVGModelObject (named after RenderBoxModelObject) is said new class.
This first patch will just add the basic plumbing. Future patches will add actual logic to this class.

(In reply to comment #2)
> (From update of attachment 29518[details] [review])
> why are SVGRenderTreeAsText and RenderTreeAsText getting a new include?
>
They got a new include because I removed the unneeded include from RednerSVGContainer.